1. The Field of the Invention
The present invention relates to the field of semiconductor lasers. More particularly, the present invention relates to systems and methods for suppressing second order modes in semiconductor lasers.
2. The Relevant Technology
Lasers are some of the primary components of optical networks. They are often used, for example, in optical transceivers to generate the optical signals that are transmitted over an optical network. Lasers are also used to pump various types of optical amplifiers, such as Raman amplifiers and erbium-doped amplifiers.
Edge-emitting lasers such as Fabry-Perot lasers, Distributed Feedback lasers (DFB lasers), and distributed Bragg reflector lasers (DBR lasers), etc., are examples of semiconductor lasers used in optical environments. Ridge waveguide lasers are a form of edge-emitting lasers that have an etched ridge.
The output spectrum of semiconductor lasers such as edge-emitting lasers is often related to the length of the laser's cavity. Because edge-emitting lasers tend to have relatively long cavities, there are several wavelengths that may lase within the cavity. As a result, many edge-emitting lasers are often referred to as multiple longitudinal mode (MLM) lasers.
The output spectrum of MLM lasers may have a spectral width of around 10 nanometers, but the spectral width can vary from one laser to the next. Although MLM lasers can be useful in various applications, MLM lasers become less useful as the speed of an optical network increases. In other words, using MLM lasers in high speed optical networks typically leads to chromatic dispersion. In addition, wavelength division multiplexing (WDM) systems experience substantial crosstalk when MLM lasers are used.
One of the ways that the spectral width of a semiconductor laser is reduced is to use distributed reflectors such as in a distributed feedback laser (DFB laser) or a DBR laser. A DFB laser or a DBR laser both typically have a spectral width that is more narrow than a simple Fabry-Perot laser. In some lasers, a mesa or ridge may also be formed by etching away part of the semiconductor laser. The ridge also helps to make the semiconductor laser emit a single mode.
In fact, the modes emitted by the laser can be affected by the width of the mesa or ridge. If the ridge is too wide, the laser may support a 2nd order transverse mode, so that two modes exist for each longitudinal mode. Unfortunately, a narrow width, which may result in a more narrow spectral width, has performance disadvantages such as reduced optical confinement that leads to higher threshold currents and higher voltages which lead to poor thermal performance. The ability to fabricate a single mode laser is also complicated by the material gain of the laser. At lower temperatures, the material gain of a semiconductor laser tends to blueshift (move to shorter wavelengths). Because the second order transverse mode of a laser is located on the blue side of the main mode of the semiconductor laser, the second order mode may affect the performance of the semiconductor laser.